J . A m . Chem. SOC.1982, 104, 5837-5839 RCH-CHC02Bz I I HO NE22
DAST THF, R T
RCH I Bz2N
- C H C 0 2 BZ I F
-I 2A 26 -
R=H
3A -
R = iPr ( t h r e o )
64
R = iPr
-
R = iPr (erythro)
&6
R = iPr
38
R=H
R = Me ( i h r e o )
5A R=Me
R = Me ( e r y t h r o )
58
R=Me
N,N-dibenzyl-a-fluoro-@-alanine benzyl ester (4, Scheme I) in 90% yield.9 The structure of 4 was established by ' H and I9F NMR, IR, and mass spectrometry. Hydrogenolysis of 4 over Pd/C 10% catalyst gave optically active a-fluoro-@-alanine, +29.1° ( c 1.05, H20).10 So that stereospecificity of the reaction could be examined, the diastereoisomeric threonine and allothreonine derivatives 2A and 2B, respectively, were subjected to the same reaction conditions (the products and yields are given in Table I). Inspection of the N M R spectra of the rearranged fluorinated products obtained indicated that only one diastereoisomer from each reaction (5A and 5B) was formed. The stereochemical purity of the products was confirmed by GLC analysis of their N-trifluoroacetyl isopropyl ester derivatives. On the basis of the N M R data of 5A and 5B and particularly the chemical shifts of the a hydrogen, we have assigned the threo configuration to the compound with the signal of higher field (5A) and the erythro configuration to the one of lower field (5B). In order to confirm this assignment, we subjected the monobenzyl derivative of 5A to X-ray analysis. The crystal structure established the compound to have the threo configuration. I The same reaction was also used with threo- and erythro-3hydroxyleucine'* derivatives (3A and 3B, respectively) in order to produce the rearranged fluorinated products 6A and 6B, respectively. The stereochemical purity of the diastereoisomers 6A and 6B was shown by HPLC analysis (over silica SI 100, with CH2C12/hexane (2/3, v/v) as eluent). The assignment of the configuration was based on N M R data (similar to the previous case): 6A, the threo configuration; 6B the erythro configuration. The stereospecificity of this reaction may be rationalized by the intermediacy of an aziridinium ion. The latter is probably formed by initial attack of DAST on the hydroxy group of the amino acidI3 and subsequent cyclization. Opening of the aziridinium ion by fluoride attack at the carbon atom a to the carboxyl group yields the rearranged product. Support for the suggested mechanism was obtained when fluorination of the D-i~oserine'~ derivative 7 with DAST provided a-fluorc-@-alaninein identical yield and optical activity as obtained from L-serine, implying a common intermediate (8). In addition, the isolation of both (2R,3R)-2-dibenzyl-amino)-3-fluorobutyric acid benzyl ester (26%) and a-fluoro-@-amino acid 5A (60%) from fluorination of N,N-dibenzyl-L-threonine benzyl ester (2A) (see Table I) strongly suggests ring opening at either the @ or the cr positions of the aziridinium intermediate.I5 ~
~~~
(8) W. J. Middleton and E. M. Bingham, Org. Synth., 57, 50 (1977). (9) A solution of 1 (1.875g, 5 mmol) in dry T H F (10mL) was added for 20 min to a stirred solution of DAST (0.72 mL, 5.5 mmol) in dry T H F (10 mL) at room temperature. The reaction mixture was stirred for a further 30 min, and then ice was added to destroy the excess DAST. The mixture was diluted with EtOAc, washed with 5% NaHCOS, and concentrated in vacuo. Chromatography of the residue on silica gel (with 3.5% EtOAc in hexane as eluent) yielded 1.750g of the rearranged fluorinated product 4. (10)The optical purity of 4 is under current investigation. (1 1) The full X-ray structure has been established by Dr. F. Frolow and will be reported in a forthcoming publication. (12) A. Shanzer, L. Somekh, and D. Butina, J . Org. Chem. 44, 3967
(19791. (13) W. J. Middleton, J . Org. Chem., 40,574 (1975). (14) Y. Shimohigashi, M. Waki, and N. Izumiya, Bull. Chem. SOC.Jpn., 52. 949 (1979). '( 15) The fomation of the 8-fluoro product with retention of configuration '
implies neighboring group participation by the dibenzylamino group. DAST fluorination is known to otherwise proceed with inversion of configuration; see T. J. Tewson and M. J. Welch, J . Org. Chem., 43, 1090 (1978).
0002-7863/82/1504-5837$01.25/0
5837
The formation of aziridinium ion intermediates in rearrangement reactions16 and the participation of a carbonyl group in enhancing SN2attack at the a-carbon atom" are known. However, no case of opening of aziridinium ions at the carbon atom adjacent to the carbonyl group has, to our knowledge, been reported. This method thus provides a useful route for the formation of a-fluoro-@-aminoacids from @-hydroxy-a-aminoacids in high stereospecificity and high yield. Further experiments are in progress to determine the detailed mechanism and stereochemical pathway of this rearrangement. Registry No. 1, 82770-40-9;Za, 82770-41-0;Zb, 82770-42-1;3a, 82770-43-2;3b, 82770-44-3;4, 82770-45-4;5a, 82770-46-5;5b, 8277047-6;6a,82770-48-7;6b,82770-49-8;7,82770-50-1;DAST, 38078-09-0; (3R,3R)-2-(N,N-dibenzylamino)-3-fluorobutyric acid, 82770-51-2. (16) D. R. Crist and N. J. Leonard, Angew. Chem.,Int. Ed. Engl., 8,962 (1969). (17) E. S.Gould, 'Mechanism and Structure in Organic Chemistry", Holt, Rinehart and Winston: New York, 1959,p 284.
Synthesis and Structure of Novel Mononuclear and Binuclear Zerovalent Platinum Complexes Involving Coordination by Tin Centers To Give Heteropolymetallic Arrays of Three or Five Atoms. A Unique Bridging Function for Bivalent Tin and a New Class of Platinum(0) Dimers Gordon W. Bushnell, Donald T. Eadie, Alan Pidcock, Anthony R. Sam, Rupert D. Holmes-Smith, and Stephen R. Stobart* Department of Chemistry, University of Victoria British Columbia, Canada V8 W 2Y2 Eileen T. Brennan and T. Stanley Cameron Department of Chemistry, Dalhousie University Halifax, Nova Scotia, Canada B3H 433 Received June 15, 1982 Metal cluster chemistry is currently an extremely active area of research. This may be attributed',* to (a) the importance of the contribution to bonding theory that has accrued through characterization of novel polyhedral geometries and (b) the potential significance in relation to catalytic applications of the incorporation into a single molecular framework of metal centers having different coordination properties. We describe representatives of two new types of heteropolymetallic species containing platinum and tin, which are related to one another by demonstrating tetrahedral coordination geometry characteristic of zerovalent platinum. Both also embody bivalent tin centers, in one structure as electron-pair donor sites in terminally bound ligating groups and in the other in a highly unusual bridging configuration, Le., as a bridging ~tannylene.~So far as we are (1) Ford, P.C.; Ungermann, C.; Laudis, V.; Maya, S. A,; Riuber, R. C.; Laine, R. H. Adu. Chem. Ser. 1979, No. 173, 225. Muetterties, E. L. Bull. Soc. Chim. Belg. 1975,84, 959. Muetterties, E. L.;Rhodin, T. N.; Band, E.; Brucker, C. F.; Pretzer, W. R. Chem. Reo. 1979, 79, 91. (2) Stobart, S. R.;Dixon, K. R.; Eadie, D. T.; Atwood, J. L.; Zaworotko, M. J. Angew. Chem., Int. Ed. Engl. 1980, 19, 931. Kraihanzel, C.S.;Sinn, E.; Gray, G. M. J . Am. Chem. SOC.1981, 103, 960. Wrobleski, D. A,; Rauchfuss, T. B. Ibid., 1982, 104, 2314. Geoffroy, G . L.; Gladfelter, W. L. Adu. Organomer. Chem. 1980, 18, 207. (3) We take the term stannylene to refer to any subvalent tin compound, Le., Sn(I1) derivative, which is monomeric and molecular in character rather than polymeric or ionic in type. Connolly, J. W.; Hoff, C. A h . Organomet. Chem. 1976, 20, 1. See also: Ewings, P. F. R.; Harrison, P. G.; King, T. J . J . Chem. Sor., Dalton Trans. 1975, 1455. Stobart, S. R.; Churchill, M. R.; Hollander, F. J.; Youngs, W. J. J . Chem. Soc., Chem. Commun. 1979, 91 I.
0 1982 American Chemical Society
5838 J. Am. Chem. Soc.. Vol. 104, No. 21, 1982
Communications to the Editor
n
L' Figure 2. Molecular structure of complex 2. Carbon atoms have been omitted for clarity. U
aware this second mode of coordination represents a unique situa tion. On addition of a hexane solution4 of Sn(acac), to (PPh,)2Pt( 7r-C2H4) in b e n ~ e n e ,the ~ reaction mixture instantly became yellow. Stirring (4 h) at ambient temperature followed by concentration and addition of hexane afforded pale orange crystals suitable for X-ray diffraction. Subsequently, an identical product (71%) was obtained through reaction of Pt(PPh,), with Sn(acac), (2 equiv) in toluene solution and was formulated as (PPh,),Pt[Sn(acac),I2 (1, mp 183-185 'C dec) on the basis of microanalytical data.6 The 31PNMR spectrum7 consisted of a singlet at 110.1 ppm flanked by satellite lines due to coupling with 195Pt ('J = 5252 Hz) and 1173119Sn (,JaV= 445 Hz); the value of 'J(Pt-P) confirms the characterization of compound 1 as a Pt(0) derivative, being in fact rather larger than those reporteds for other zerovalent platinum complexes, including Pt(PPh,), (n = 2-4). The II9Sn N M R spectrum comprised a triplet (6 '19Sn -601) with triplet sidebands [1J(119Sn-195Pt) = 12891 Hz] and further weak satellites attributable to 117Sn-119Sncoupling ('J = 4517 Hz). These data establish that in solution two tin-containing ligands remain attached to F't in 1 and imply bivalent9 character for the tin nuclei. The 195PtN M R spectrum showed a triplet structure [with appropriate satellites arising from lJaV(117J19Sn-195Pt)], proving that two phosphine ligands also coordinate the Pt center so that the complex is a four-coordinate species in solution. The molecular structure'" of complex 1 is shown in Figure 1. No closely related species exists in platinum group metal chemistry,
the zerovalent platinum center being stabilized by metal-metal interaction via electron donation from tin, which is itself subvalent in character. As such, the complex can be viewed as a highly reduced heteropolymetallic aggregate. It also joins a very limited range of compounds in which the geometry expected for fourcoordinate dl0 Pt(0) has been confirmed crystallographically,11-13 with coordination by two PPh3 groups and two Sn(acac), donors, as adduced from the NMR data, distributed around the central Pt atom in a distorted tetrahedral arrangement. The mean Pt-Sn distance is 2.558 (1) A; the Pt-P bond length at 2.261 (4) A is substantially shorter than those found in related four-coordinate complexes, being close to Pt-P in143'5Pt(PPh,), or P ~ ( P - ~ - B U , P ~ ) ~ ; the P-Pt-P angle is wide (120.1') despite the size of the tincontaining ligands. Reaction of (PPh,),Pt(?r-C,H,) with Sn(acac)2 (2 mol equiv) in refluxing toluene resulted in gradual deepening in color of the mixture; cooling after 6 h and concentration afforded a deep orange crystalline complex (2, 69%, decomposition point 92 "C). This compound was also isolated after refluxing compound 1 in toluene for 6 h. On the basis of "P, I19Sn, and '"Pt NMR spectroscopy the product was tentatively formulated as a binuclear platinum species possessing high molecular symmetry with coordination at Pt(0) by a single phosphine ligand and bridging Sn(I1) units;16 exact characterization of such a highly unusual arrangement was sought by using X-ray crystallography. The molecular structureI7 (organic fragments omitted for clarity), which is shown in Figure 2, consists of a trigonal bipyramid with the Pt atoms separated from one another by 2.593 (1) A and with bond angles P-Pt-Sn = 118.9O and Sn-Pt-Sn = 98.6'. The two apical coordination positions at Pt are occupied by PPh3 molecules
(4) 2 mol equiv based on Pt; acacH = acetylacetone. Sn(acac), was synthesized as described elsewhere: Bos, K. D.; Budding, H. A.; Bulten, E. J.; Noltes, J. Inorg. Nucl. Chem. Lett. 1973, 9, 961. ( 5 ) (PPh3),Pt(?r-C2H4) was prepared by using the published procedure: Blake, D. M.; Roundhill, D. M. Inorg. Synth. 1978, 18, 120. (6) The IH NMR spectrum (C&) contained signals at T 3.2-3.8 (15, m), 5.76 (2, s) and 8.70 (12, s) attributable to C6H,, CH, and CH, protons, respectively, and the IR spectrum (KBr pellet) showed bands near 1600 cm-' due to v(C=O) and at 1430, 1480 cm-I, characteristic of PPh3. (7) All NMR spectra were recorded by using a Bruker WM-250 s ec trometer operating at 101.27, 93.28, and 53.27 MHz for ,'P, lI9Sn, and 5Pt, respectively. Values for J a r e f 3 H z for "P and f 1 2 Hz for the two metals. Chemical shifts are in ppm vs. external P(OMe)3 ()IP) and SnMel (*I9Sn). (8) Pregosin, P. S.; Kunz, R. W. "31Pand 13CNMR of Transition Metal Phosphine Complexes"; Springer-Verlag: Berlin, 1979. (9) For Sn(acac), we observe 6 '19Sn -702. See also: Kennedy, J. D.; McFarlane, W. Reu. Silicon, Germanium, Tin Lead Compd 1974, l ?235. Smith, P. J.; Smith, L.Inorg. Chim. Acta Reu. 1973, 7 , 11. (10) Crystal data for 1: CS6H5808P2Sn2Pt; mol wt 1353.4; orthohombic; space group Pca2,; a = 19.327 (5) A, b = 12.811 (2) A, c = 21.978 (2) A; Z = 4; p(ca1cd) = 1.649 g cm-l; Mo K a radiation, X = 0.71069 A; Nonius CAD4 diffractometer; R = 0.033 for 1967 observed reflections, I > u ( I ) .
(1 1) Albano, V. G.; Bellon, P. L.; Manassero, M. J . Organomet. Chem. 1972, 35, 423. (12) Albano, V. G.; Bellon, P. L.; Manassero, M. Chem. Commun. 1969, 899. (13) Eller, P. G.; Ryan, R. R.; Moody, D. C. Inorg. Chem. 1976, IS, 2442. Linsky, J. P.; Pierpont, C. G. Ibid. 1973, 12, 2959. (14) Albano, V.; Bellon, P. L.; Scatturin, V. Chem. Commun. 1966, 507. (15) Otsuka, S.; Yoshida, T.; Matsumoto, M.; Nakatsu, K. J . Am. Chem. Soc. 1976, 98, 5850. (16) Full analysis of the NMR data for compound 2 is obviously lengthy and will be described in detail elsewhere. In summary the 195Ptspectrum shows coupling to only one nucleus with ' J = 5545 Hz while the Il9Sn spectrum consists of triplets due to coupling to two equivalent "P nuclei ('J = 168 Hz); solution of the 195Ptspectrum also establishes the existence of Pt-Pt coupling. (17) Crystal data for 2: C66H72012P Pt2Sn,;mol wt 1865.5; trigonal; space a = 60.56 (2)"; V = 3682 (3) A,; group RJc(No. 167); a = 17.260 (6) Z = 2; p(calcd) = 1.68 g cm?; Mo Ka radiation, X = 0.71069 A; Picker diffractometer; R = 0.0399 for 1150 observed reflections. An absorption correction was calculated by numerical integration. Data collection was to 2% = 40°, with 50-s step scans and equal time on background counting.
Figure 1. Molecular structure of complex 1. Phenyl rings have been omitted for clarity.
r-
A;
J. Am. Chem. SOC.1982, 104, 5839-5841
5839
with Pt-P = 2.224 (4) A while the three equatorially bridging Effectively Hypervalent First-Row Molecules. 1. Octet ligands are symmetrically disposed with Pt-Sn = 2.680 (1) A and Rule Violations by OLiJ and OLi, Pt-Sn-Pt = 57.87 ( 3 ) O . This beautiful geometry represents the Paul von Ragui Schleyer* and Ernst-Ulrich Wiirthwein face-sharing of two distorted tetrahedral PPtSn, fragments, which constitute a series with the P2PtSn2configuration (1) and with Institut f u r Organische Chemie der the corresponding P3PtSn species (3), which we have also identified Friedrich- Alexander- Universitat using N M R spectroscopy. The two Pt atoms are within bonding Erlangen- Niirnberg, 0-8520 Erlangen distance,'* and the strong coupling between them ['J(Pt-Pt) = Federal Republic of Germany 4663 Hz] is consistent with some degree of metal-metal bonding,I9 as is 2J(P-Pt) = 82 and 3J(P-P) = 81 Hz. As for complex 1, John A. Pople nonplanar coordination is characteristic of Pt(0) while the 'I9Sn Department of Chemistry, Carnegie- Mellon University chemical shift (-563 ppm) is indicative of Sn(I1). The narrow Pittsburgh, Pennsylvania 15235 angle at the latter and the lengthening of Pt-Sn (0.12 A) between Received June 11, 1982 compounds 1 and 2 incline us toward an interpretation based on delocalization of electron density between the five metal centers. We have discovered calculationally a large number of AX,, Expressed in simple qualitative terms, this situation corresponds molecules comprised of first-row elements whose unusual stoito interaction between three hybrid orbitals on each Pt with a single chiometries suggest violations of the octet rule.' The equilibrium hybrid per Sn to give three bonding, three nonbonding, and three geometries of these species involve polycoordinated central atoms, antibonding levels. The Pt(0) nuclei then retain "closed-shell" A, which therefore appear to have some hypervalent character. dIo character while the Sn(I1) atoms each contribute an electron This communication deals with preliminary results on two such pair to fill the three delocalized bonding orbitals. Superficially molecules, OLi3 and OLi4. The former is known experimentally; there is a resemblance between structure 2 and that of gas-phase energies are available but not the geometry.2 [Me2SnFe(CO)4]2,but the latter clearly incorporates quadrivalent The octet rule is very effective in predicting the optimum tin and accordingly the angle at Sn is almost tetrahedra120 with stoichiometry of AX, first-row molecules if X is hydrogen or an that at Fe quite narrow (77.4'). The electron count in another element (or group) toward the right of the periodic table. Thus, Sn(1V) derivative2' [((~S-C,Hs)Fe(CO)2]2Sn2Fe3(CO)9] is exact addition of a hydrogen atom to an oxygen atom to give the OH in terms of isolobal relationshipsz2 for a closo cluster, but deloradical is exothermic by 101 kcal/mol; addition of a second hycalized metal-metal bonding tentatively proposed in a somewhat drogen atom to give H 2 0 is even better, releasing 118 k ~ a l / m o l . ~ similar platinum-tin(1V) cluster23 [(C8H12)3Pt3(SnC13)2] does not However, the hypervalent species OH3 and OH4 appear to be only appear to have been discussed further. A more realistic molecular very weak van der Waals complexes between H 2 0 and H or H2.4 orbital description of the intermetallic bonding in compound 2 If the bonded atom X is lithium rather than hydrogen, the will necessarily take account of interaction involving overlap of situation is completely different. To illustrate this, we consider unfilled orbitals of comparable energy with the Pt filled-shell orbitals, similar to that put forward by Dedieu and H ~ f f m a n n ~ ~ the compounds OLi, for n = 1-4. Table I gives calculated dissociation energies for OLi,, OLi,, + Li. (These results were for unbridged Pt(0)-Pt(0) complexes of the type Pt2L4. obtained by minimizing the Hartree-Fock/3-2 lG5 energy to give A comprehensive investigation of the properties of compounds the structure and then using the larger 6-31G* basis6 for MP2 typified by 1-3 is in progress, including development of related correlated calculations7at these geometries. Zero-point corrections Ni and Pd chemistry. Since methanolic solutions containing were made by using harmonic frequencies from the HF/3-21 G chloroplatinic acid and stannous chloride provided the first effective (see Table 11). Clearly lithium does not respect the octet catalyst for the reduction of ethylene under ambient condition^^^ rule at all! The sequential addition of lithium atoms continues and polynuclear Pt-Sn species have been implicatedz6 in similar perfectly well after the second step and both OLi, and OLi4 are behavior, the possibility that the Pt(O)/Sn(II) systems will demindicated to be quite stable thermodynamically toward dissociation onstrate parallel catalytic reactivity is being explored. or loss of an electron. In further work to be described subsequently, OLi, and OLi6 are also indicated to be stable molecules. Acknowledgment. We thank the NSERC, Canada, for operating grants (G.W.B., S.R.S., and T.S.C.) and the University of ( I ) Lewis, G. N . J . Am. Chem. SOC.1916,38,762. Kossel, W. Ann. Phys. Victoria for financial support, including graduate fellowships to (Leipzrg) 1916, 49, 229. Pauling, L. J . Am. Chem. SOC.1931, 53, 1367. D.T.E. and R.D.H.6.; A.P. thanks the University of Sussex for Kuznetsov, V. I., Ed. "Theory of Valency in Progress"; MIR Publishers: leave of absence. MOSCOW, 1980; p 112 f, 123 f. Perkins, C. W.; Martin, J. C.; Arduengo, A.
-
Registry No. 1, 82918-18-1; 2, 82918-19-2; Sn(acac),, 16009-86-2; (PPhj)*Pt(~-c*H4), 12120-1 5-9; Pt(PPhJ4, 14221-02-4; Pt, 7440-06-4; S n , 7440-31-5.
Supplementary Material Available: Listings of crystallographic data for PtSn2(acac),(PPh3), and Pt,Sn,(aca~)~(PPh,)~ (29 pages). Ordering information is given on any current masthead page. (18) Yoshida, T.; Yamagata, T.; Tulip, T. H.; Ibers, J. A,; Otsuka, S. J . Am. Chem. SOC.1978, 100, 2063. (19) Boag, N . M.; Browning, J.; Crocker, C.; Goggin, P. L.; Goodfellow, R. J.; Murray, M.; Spencer, J. L. J. Chem. Res., Synop. 1978, 228. (20) Gilmore, C. J.; Woodward, P. J. Chem. SOC.,Dalton Trans. 1972, 1387. (21) McNeese, T. J.; Wreford, S. S.;Tipton, D. L.; Bau, R. J . Chem. SOC., Chem. Commun. 1977, 390. (22) Elian, M.; Hoffmann, R. Inorg. Chem. 1975, 14, 1058. Elian, M.; Chen, M. M.; Mingos, D. M. P.; Hoffmann, R. Ibid. 1976, 15, 1148. Jemmis, E. D.; Hoffmann, R. J . Am. Chem. SOC.1980, 102, 2570. (23) Guggenberger, L. J. Chem. Commun. 1968, 512. (24) Dedieu, A,; Hoffmann, R. J . Am. Chem. SOC.1978, 100, 2074. (25) Cramer, R. D.; Jenner, E. L.; Lindsey, R. V.; Stolberg, U. G. J . Am. Chem. SOC.1963, 85, 1691. (26) Bond, G. C.; Hellier, M. J . Catal. 1967, 7, 217. Pietropaolo, R.; Dolcetti, G.; Giustiniani, M.; Belluco, U. Inorg. Chem. 1970, 9, 549.
0002-7863/82/1504-5839$01.25/0
J.; Lau, W.; Alegria, A.; Kochi, J. K. J. Am. Chem. SOC.1980, 102, 7753. (2) Wu, C. H.; Kudo, H.; Ihle, H. R. J . Chem. Phys. 1979, 70, 1815. Kimura, H.; Asano, M.; Kubo, K . J. Nucl. Mater. 1980, 92, 221. It is likely that OLi4 has now been observed as well (Wu, C.H.,private communication). Weakly bound lithium atom complexes with water (and other bases) have been investigated calculationally (Trenary, M.; Schaefer, H. F., 111; Kollman, P. J. Am. Chem. SOC.1977, 99, 3885; J. Chem. Phys. 1978, 68, 4047) and experimentally (Meier, P. F.; Hauge, R. H.; Margrave, J . L. J . Am. Chem. SOC.1978, 100, 2108). Also see ref 15. (3) Rosenstock, H. M.; Draxl, K.; Steiner, B. W.; Herron, J . T. J. Phys. Chem., ReJ Data Suppl. 1977, IC. Huber, K. P.; Herzberg, G. "Constants of Diatomic Molecules"; Van Nostrand Reinhold: New York, 1979. (4) Kari, R. E.; Csizmadia, I. G. J . Am. Chem. SOC.1977, 99, 4539. Niblaeus, K. S. E.; Roos, B. 0.;Siegbahn, P. E. M. Chem. Phys. 1977, 25, 207. (5) The GAUSSIAN 76 (Binkley, J. S.; Whiteside, R. A.; Hariharan, P. C.; Seeger, R.; Pople, J. A.; Hehre, W. J.; Newton, M. D. QCPE, 1979, 1 I , 368) and the GAUSSIAN 80 CMU (Binkley, J. S.; Whiteside, R.; Krishnan, R.; Seeger, R.; Schlegel, H. B.; DeFrees, D. J.; Topiol, S.;Kahn, L. R.; Pople, J. A. QCPE 1981, 13, 406) programs were employed. For the 3-21G basis set, see: Binkley, J. S.; Pople, J. A.; Hehre, W. J. J . Am. Chem. SOC.1980, 102, 939. (6) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (7) Binkley, J. S.; Pople, J . A. Inf. J . Quantum Chem Symp. 1977, 99, 4899. (8) Pople, J. A.; Krishnan, R.; Schlegel, H. B.; Binkley, J. S. Int. J. Quantum Chem. Symp. 1979, 13, 325. (9) Pople, J. A.; Schlegel, H. B.; Krishnan, R.; DeFrees, D. J.; Binkley, J . S.; Frisch, M. J.; Whiteside, R. A,; Houk, R. J.; Hehre, W. J. Proc. Sanibel Symp., in press.
0 1982 American Chemical Society
